1. Field of the Invention
This invention relates to a low alloy, heat-treatable carbon steel for hydrogen sulfide service and having improved hardenability.
2. The Prior Art
Materials specifications of low alloy steels for arctic service in the petroleum industry impose exacting requirements. In some instances, components made from such steels must be suitable for hydrogen sulfide service. Hydrogen sulfide can embrittle steel components if the components either are not properly heat-treated or if the components are formed of low alloy steels containing more than one percent (1%) nickel. Steel components, for artic service in the petroleum industry, must exhibit a relatively high minimal toughness, as measured by a charpy V-notch test, to withstand expected impulsive loads without failure. Due to the low temperatures often encountered in the arctic, various production equipment purchasers have specified that charpy impact tests be performed on specimens having a temperature of approximately minus 75.degree. F. Generally, the specifications of equipment purchasers specify that a set of three charpy V-notch specimens, when broken at the designated test temperature, show a minimum average impact value of fifteen (15) foot pounds with no single value below either ten (10) or twelve (12) foot pounds.
Using present commercially available low alloy steels, these charpy impact values cannot be reliably obtained at the center of a three-inch thick cross-section. A three-inch thick cross-section would be typical for a flange or other heavy walled part.
For a quenced and tempered low alloy steel, notch toughness depends upon chemical composition and micro-structure. Microstructure in turn depends upon the thickness of the component at the time of heat treatment, the severity of quench (measured by H value) and hardenability (measued by D.I. factor). Presently available low alloy commercial steels do not have a sufficiently high minimum hardenability and the ability to undergo a severe quench. These steels cannot be heat-treated to develop the micro-structure, at the center of a three-inch thick section, which would reliably produce the relatively high specified minimum impact values. For example, although A.I.S.I standard alloy steel 4130 can undergo a rather severe water quench (H=1.5), 4130 steel has too low of a hardenability (nominal D.I. of 4130 steel=2.79") to attain the desired micro-structure at the center of a three-inch section. On the other hand, A.I.S.I. standard alloy steel 4140, which has a higher hardenability (nominal D.I.=4.79), cannot undergo a severe quench. The higher nominal carbon content of 4140 steel and its higher D.I. value increase its susceptibility in cracking. In fact, 4140 steel must be oil quenched, for H=0.5, to avoid cracking. The inability to severely quench 4140 steel prevents the development of the proper micro-structure within the center of a three-inch thick section.